The present invention relates generally to crystal manufacture methods and apparatuses, and more particularly to a manufacture method for calcium fluoride (CaF2) crystal usable as a material for optical elements used for an exposure apparatus for photolithography.
Along with the recent demands on smaller and thinner-profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern tries to achieve a line and space (L & S) of 130 nm on a mass production line, and predictably it will be increasingly smaller in the future. L & S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
The exposure has three important parameters, i.e., resolution, overlay accuracy and throughput. The resolution is a minimum size for a precise pattern transfer. The overlay accuracy is a precision with which to overlay multiple patterns over an object to be exposed. The throughput is the number of sheets exposed per unit of time.
A shortened wavelength of exposure light and a larger aperture of a projection lens are effective for improved resolution. For the shortened wavelength, a light source has been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) to ArF excimer laser (with a wavelength of approximately 193 nm), and F2 excimer laser (with a wavelength of approximately 157 nm) is about to reduce to practice. On the other hand, a lens with a diameter of 250 mm or larger has been developed for the larger aperture.
The CaF2 crystal has higher light transmittance (or internal transmittance) for this wavelength range than other glass materials, and thus is the best as an optical material for optical elements, such as a lens and a diffraction grating, applicable to an exposure optical system.
Parameters for evaluating optical materials for a lens, etc. include, in addition to the internal transmittance, laser durability indicative of transmittance changes in response to continuous receptions of a laser beam, refractive index homogeneity indicative of a degree of constant of a lens's refractive index according to positions, birefringence, process or (grinding) accuracy, etc.
A method for manufacturing CaF2 crystal with good one or more optical characteristics among them has been already disclosed in Japanese Laid-Open Patent Applications Nos. 9-315893 and 10-330192.
However, the CaF2 crystal manufactured by the conventional manufacture methods exhibit satisfactory optical characteristics for visible light, but has a disadvantage in that it has not yet exhibited satisfactory optical characteristics for short-wavelength and high-power light such as excimer laser, e.g., low laser durability, large refractive index homogeneity and birefringence, poor surface precision at the time of grinding process, etc.
As a result that the instant inventor has eagerly studied the causes, he has found that it results from the crystal or lattice defect or dross inclusion. The dross inclusion is generally removed through segregation from one-way cooling and crystallization in the purification process.
However, the crystal for use with the excimer laser has such a large aperture that it has a difficulty in one-way crystallization and cannot sufficiently remove impurities disadvantageously.
This is because as the crystal aperture becomes large, a ratio of thickness to aperture diameter becomes small, causing the uniform melt temperature distribution during melting, or the small temperature difference between top and bottom surfaces in the melt, as shown in FIG. 4. Although FIG. 4 shows a temperature distribution with a ratio of thickness to aperture diameter of 0.15, the dimension in FIG. 4 does not accord with the description for simplicity purposes.
The temperature at the bottom surface of a skull crucible containing the melt is adapted to be lower than that at the top surface. The purification process is implemented at considerably higher temperature than the melting point due to dehydration and degas, and does not use a seed that is usually used for monocrystal growth.
Therefore, impurities of relatively light specific gravity gathering at the top surface in the melt becomes a starting point for crystallization or nucleus, resulting in crystallization from the top surface as well as from the bottom surface in the melt at a low temperature. As a result, impurities, such as rare earth, and bubbles concentrate in the middle of crystal, which is a part finally solidified, and are hard to be removed.
Table 1 shows impurity data in CaF2 crystal, which is magnesium as an example. It is understood that the middle has higher magnesium concentration than the top and the bottom. In other words, magnesium accumulates in the center.
TABLE 1Mg CONTENTTOP PART0.55MIDDLE PART1.00BOTTOM PART0.68